1. Field of the Invention
This disclosure relates to the field of generation of solutes in equilibrium with their vapor phase. These are commonly used as wet bath calibration systems such as to calibrate breath alcohol testing equipment.
2. Description of the Related Art
For the purposes of public safety on the roads and elsewhere, there is a need to make sure that individuals are not operating potentially dangerous machines (such as automobiles) while they are impaired by the effects of alcohol consumption. To try and prevent people from driving drunk, most states have enacted laws that impose fines or other criminal penalties if individuals have a breath or blood alcohol level above a certain amount. In order to effectively enforce these laws, it is necessary to be able to measure the alcohol concentration in human breath (which is often used as an easy method for approximating blood alcohol if it is not used directly) and compare the results against legal limits. There are a variety of measuring instruments used for determining the concentration of alcohol in human breath ranging from small hand held devices to larger bench top units and machines built into cars or certain machinery. Since a determination of breath alcohol above the legal threshold can result in criminal penalties, loss of a job, or other sanctions, the accuracy of these instruments is paramount.
Great care and effort is taken by owners and managers of evidential breath testing equipment to ensure proper calibration as well as follow-up accuracy checks at generally regular intervals. In attempts to eliminate the labor time of this testing and concerns about human error in the testing, manufacturers of breath testing equipment often offer automated or semi-automated methods for doing calibrations and accuracy checks. Some users of breath alcohol test equipment, such as Motor Vehicle Law Enforcement, may even require an automatic accuracy check every time they test a human subject and sometimes even before and after the human subject test simply to make sure that the device is reading correctly and will supply court-admissible evidence.
There are generally different standards used when calibrating breath testers. As breath (containing alcohol or not) is a vapor comprising exhalation gases and vaporized substances and can be quite complex, instruments that measure alcohol concentration in breath vapor generally need standards to be provided in a similar form for accurate calibration. Calibration gases of many sorts are well known in many applications including breath testing. In breath testing, the calibration standards are generally of two types, wet and dry. Wet standards include water vapor; dry standards do not. Some argue that wet standards are better because they include moisture like human breath and are therefore more representative. Effectively, the argument is that the closer the calibration gas is to the actual constitution of an expected human breath at the tipping point of legal consequences, the more robust the calibration is, and the more likely that an evidentiary reading will be determined to be “correct” in the end. However, commercial providers of both wet and dry standards generally advertise+/−2% accuracy of calculations with actual breath.
In either case, the alcohol concentration of measurement interest is in a carrier gas such as air, breath, or nitrogen. A typical breath ethanol concentration which would result in illegal driving in most states is 200 parts per million (ppm) or more. That is 200 parts ethanol per million parts of carrier gas regardless of the carrier gas composition. Therefore, the standards generally provide samples that contain very close to 200 ppm to make sure the dividing line is correctly calibrated.
Wet standards have a long history in breath testing, are well accepted, and the liquids used in them can be certified by chemical analysis against National Institute of Standards and Technology (NIST) traceable standards. The standards are prepared by combining known amounts of ethanol and water in a partially filled jar that is accurately heated (generally to 34° C.) and then maintained at that temperature. These heated jars are sold commercially and are referred to as Simulators. At equilibrium, the quiescent headspace above the jar contains a vapor with a known concentration of ethanol along with nearly 100% relative humidity at that temperature. In one special case of a wet standard, known as an “Equilibrator,” no heating is used, but the operator is required to read its temperature (usually equal to ambient) and follow a lookup table to see what gas concentration is delivered when similarly blown through as in a standard simulator.
By introducing sober human breath or air from another suitable source into the jar (by blowing or injecting gas into the liquid), the known concentration of ethanol vapor exits the headspace and can be introduced into a breath tester at which point a measurement may be taken. Typically, a liter or more of gas is blown through the simulator for each test. As newly introduced air or breath bubbles up through the liquid, it replaces the gas exiting the simulator with newly equilibrated gas.
Generally, the simulators of the prior art go to great lengths to keep the temperature of the system constant at 34° C.+/−0.1° C. This is because, as the temperature changes, so does the equilibrium point. Thus, the alcohol concentration in the gas varies with the temperature of the system. For example, at 34° C., a 0.1° change can represent well over a 0.5% change in the gas. Notably, this air/water equilibrium relationship for ethanol over temperature is not linear. Those skilled in the art will recognize, as shown in table 1 below that the ratio of ethanol concentration in the air to the water goes up in a non-linear fashion as the temperature goes up:
TABLE 1° C.KA/W × 10310.03550.046100.073150.107200.155250.217300.310350.418370.470400.562
Thus, while these types of wet calibration systems are well established in the art, there can be no question that there is some concern about their specific ability to provide a highly accurate sample in today's demanding environment for repeatable calibration and testing of measurement devices. Prior breath testing devices were commonly allowed a +/−10% accuracy in their readings. As the error introduced from the calibration sample was relatively small compared to this, it was of relatively little concern as most error would exist in the breath testing device itself. However, today's standards are much more rigorous and most breath testing instruments are allowed, at most, a +/−5% margin of error and are often allowed much less. These specifications can be extraordinarily hard to meet when a calibration standard can, by itself, introduce up to two-thirds of the allowable error. In effect, the breath testing devices are effectively allowed to have less variation than the standards they are tested against in order to pass.
While there are some arguments that this simply makes the resultant systems all the more accurate, it does make clear that even if all procedures for accurate calibration of the device are followed and the device is infinitely accurate, there has always been a degree of uncertainty in the accuracy of the gas used to calibrate the device. This high degree of uncertainty in the standard has become one of the biggest components in uncertainty in the instruments and has, in some respects, resulted in a barrier to the recognition that instruments may be even more accurate than they currently appear because they simply cannot be accurately calibrated. Thus, there is a need in the art for calibration systems which reduce their contribution in the uncertainty of the testing equipment and therefor allow for recognition as to the actual accuracy of the testing equipment itself.